In-situ element geochemical and sulfur isotope signature of pyrite and chalcopyrite: Constraints on ore-forming processes of the Laoshankou iron oxide-copper (-gold) deposit, northern East Junggar
In-situ element geochemical and sulfur isotope signature of pyrite and chalcopyrite: Constraints on ore-forming processes of the Laoshankou iron oxide-copper (-gold) deposit, northern East Junggar
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DOI：
10.1016/j.oregeorev.2021.104510
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发表时间：
2021-10
影响因子：
3.3
通讯作者：
Pei Liang;Li Chen;Rucao Li;Yuling Xie;Chao Wu;Chunkit Lai
中科院分区：
文献类型：
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作者：
Pei Liang;Li Chen;Rucao Li;Yuling Xie;Chao Wu;Chunkit Lai
老山口存款是位于中亚造山带东准噶尔北方边缘的一个类似铁氧化物-铜-金（IOCG）的存款矿床，黄铁矿普遍存在于II期磁铁矿化（Py 1a和Py 1b）、III-A期黄铁矿化（Py 2a和Py 2b）和III-B期黄铜矿化（Py 3a和Py 3b）。尽管前人对老山口存款矿床的成矿流体来源进行了大量的研究，但仍存在争议。近年来，IOCG系统中是否有非岩浆流体的参与也引起了很大的争议。为进一步研究古生代盆地反转背景下IOCG矿床的成矿流体性质，对老山口存款中一组黄铁矿/黄铜矿样品进行了原位二次离子质谱（西姆斯）硫同位素和激光烧蚀电感耦合等离子体质谱（LA-ICP-MS）微量元素分析。各期硫化物的硫同位素结果（-3.5 ~2.6 ‰）均符合岩浆硫范围，表明岩浆硫的贡献。但δ 34 S值为-3.5 ‰ ~-1.5 ‰（Py 3），呈弱负值，可能是有机硫的加入，这也可从富有机质流体包裹体和地层中得到证实。低Se/S（<5 × 10−5）、类似火山成因的Co/Ni比值（3.9-28，平均16）和不同的Ni含量（81-978 ppm，平均315 ppm）表明Py 1的岩浆硫在第II阶段被加热的海水从镁铁质火山岩中浸出。Py 2的Ni（433-3736 ppm，平均1401 ppm）和Se（39-143 ppm，平均80 ppm）含量升高，Se/S（≤ 1.5 × 10−4）比表明更氧化的条件。我们认为，Py 2在第III-A阶段的岩浆硫也被海水从基性岩中浸出，但不能排除岩浆热液输入。Py 3的高Co/Ni（7.7-415，平均142）和低Ni/Se（0.43-3.4，平均1.7）比值以及低Ni含量（31-381 ppm，平均133 ppm）表明，这些有机硫可能是由相对低温和还原流体从富有机质序列中浸出的，其中III-B阶段的长英质岩石贡献显著。结合前人的卤素和C-H-O同位素研究结果，证明老山口存款矿床存在非岩浆流体。第III-B阶段黄铜矿颗粒取代了早期黄铁矿，在此期间，一些据称流动性较低的元素（例如，Co、As释放到Cu成矿流体中，形成了极低Co、多孔/裂隙、富含包裹体的Py 1b和Py 2b，同时在Py 3和Py 3-黄铜矿边界形成了高Co-As含量的Py 1b和Py 2b。外海水和有机盐卤可能对老山口的成矿作用有影响。结合前人在安第斯山脉和东天山的研究，外流体可能对弧相关盆地反转背景下的IOCG沉积起着关键作用。
The Laoshankou deposit is an iron oxide-copper–gold (IOCG)-like deposit in the northern margin of East Junggar, Central Asian Orogenic Belt (CAOB), in which pyrite is ubiquitous and present from stage II magnetite mineralization (Py1a and Py1b), stage III-A pyrite mineralization (Py2a and Py2b), to s tage III-B chalcopyrite mineralization (Py3a and Py3b). Despite of many previous studies, the source(s) of ore-forming fluids in the Laoshankou deposit are still controversial. Whether non-magmatic fluids were involved in the IOCG systems has also been highly debated in recent years. To further characterize the nature of ore-forming fluids in Paleozoic IOCG deposits in basin inversion settings, we conducted in-situ secondary ion mass spectrometry (SIMS) sulfur isotope and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analysis in a set of pyrite/chalcopyrite-bearing samples in the Laoshankou deposit. Sulfur isotope results of the sulfides in all stages (−3.5 to 2.6‰) fit in the magmatic sulfur range, indicating the contribution of magmatic sulfur. However, the weakly negative δ34S values of − 3.5‰ to − 1.5‰ (Py3), possibly indicating the addition of organic sulfur, which can also be proved by organic-rich fluid inclusions and strata. The low Se/S (<5 × 10−5), the volcanogenic-like Co/Ni ratios (3.9–28, average 16), and the varying Ni contents (81–978 ppm, average 315 ppm) of Py1 indicate that its magmatic sulfur was leached from mafic volcanic rocks by heated seawater in a reducing condition at stage II. The elevated Ni (433–3736 ppm, average 1401 ppm) and Se (39–143 ppm, average 80 ppm) contents, and the Se/S (∼1.5 × 10−4) ratios of Py2 indicate a more oxidizing condition. We suggest that the magmatic sulfur of Py2 in stage III-A was also leached from mafic rocks by seawater, but magmatic hydrothermal fluid input cannot be excluded. The high Co/Ni (7.7–415, average 142) and low Ni/Se (0.43–3.4, average 1.7) ratios, and the low Ni contents (31–381 ppm, average 133 ppm) of Py3 show that such organic sulfur was likely leached from organic-rich sequence by relatively low-temperature and reduced fluid, with significant contribution from the felsic rocks in stage III-B. Combined with previous halogen and C–H-O isotopes results, external non-magmatic fluids can be proved in the Laoshankou deposit. Stage III-B chalcopyrite grains replaced early-stage pyrites, during which some alleged less-mobile elements (e.g., Co, and As) in the replaced pyrites were released into the Cu ore-forming fluids and formed extremely low-Co, porous/fractured, and inclusion-rich Py1b and Py2b, together with high Co-As contents in the Py3 and the Py3-chalcopyrite boundary. We propose that external seawater and organic bittern brine may have an effect on the mineralization at Laoshankou. Combined with previous researches in the Andean and East Tianshan, external fluids may play a key role for the IOCG deposits in arc-related basin inversion setting.